(a) Field of the Invention
The present invention relates to an optical semiconductor device and a method of manufacturing the same and, more particularly, to the technique for forming a semiconductor laminate mirror, which has a high reflectance and/or surface refractive anisotropy, in a semiconductor laser device such as a surface emission laser, optical cavity and photodetector.
(b) Description of the Related Art
A semiconductor laminate mirror is generally used in an optical semiconductor device such as a surface emission laser device. To fabricate the semiconductor laminate mirror by a known epitaxial process, it is necessary to form two kinds of thin films in pairs constituting the semiconductor laminate mirror from materials of substantially the same lattice constant as that of the laser active layer, i.e., from materials lattice-matched with the laser active layer.
In the semiconductor laminate mirror, it is desirable to incorporate materials providing a higher reflectance by a less number of layers to reduce optical absorption by the mirror. To achieve this, a higher ratio needs to be set between the refractive indices of the two rinds of thin semiconductor films, one having a high refractive index and the other having a low refractive index, both constituting the semiconductor laminate mirror.
In the meantime, polarization control in the surface emission laser device is extremely important in view of the polarization dependency of the current optical devices. To achieve an anisotropy in polarized light emitted by the surface emission laser device, the prior art teaches the following methods:
(i) emitted light is subjected to an anisotropic optical loss; PA1 (ii) an elliptical hole is formed in the laminate to apply an anisotropic stress thereto; PA1 (iii) a diffraction grating is provided on the laminate mirror; PA1 (iv) anisotropy is provided in the horizontal structure of an optical oscillator itself; and PA1 (v) anisotropic matrix elements substituting in a strained quantum well are used.
The methods (i) to (iii) as mentioned above more complicate each fabrication process, the method (iv) causes anisotropy to the light distribution in case of a surface emission laser device, and the method (v) may allow no independent design for the active layer from the design for the anisotropy.
Prior art does not teach any combination of materials providing a sufficiently high ratio of the refractive indices (i.e., relative refractive index) between the InP substrate and materials lattice-matched with the InP substrate. By contrast, such a combination is known in the case of a GaAs substrate wherein the GaAs substrate and materials lattice-matched with the GaAs are used. Accordingly, a direct bonding technique is generally used in the prior art surface emission laser device to lase at 1.3 .mu.m or 1.55 .mu.m, wherein an epitaxial wafer grown on an InP substrate for a surface emission laser device is bonded to a high reflection laminate mirror made of GaAs/AlAs thin film pairs grown on a GaAs substrate.
The GaAs/AlAs films as described above, however, combine to create unsatisfactory design flexibility for the surface emission laser devices. Moreover, it is difficult to achieve a desired higher relative refractive index in such a combination, setting limits on the improvement in reflectance of the laminate mirror.